This invention relates to radio frequency dual band combiners, for combining signals into two different frequency bands, for example, 900 MHz and 1800 MHz, for connection to a single feeder cable via a common port of the combiner.
A combiner of the aforementioned type must have, on the one hand, high isolation between the two frequency bands, and on the other hand, a low insertion loss.
Such requirements are difficult to implement because high isolation demands elaborate filters, and elaborate filters lead to increased insertion loss.
High isolation combiners using coaxial bandpass filters are known, but their insertion loss is usually unattractively high which reduces overall system performance. Bandpass-band reject combiners are known. However, the insertion loss in the bandpass branch is relatively high in most cases.
If a combiner is to be realized in planar structure, i.e. using microstrip filters, then maximum resonator Q is very low compared to coaxial resonator bandpass filters, which also leads to increased insertion loss.